A liquid transfer article, such as an impression roll, is used in the printing industry to transfer a specified amount of a liquid, such as ink or other substance, from the liquid transfer article to another surface. The liquid transfer article generally comprises a surface with a pattern of depressions or wells adapted for receiving a liquid and in which said pattern is transferred to another surface when contacted by the liquid transfer article. When the liquid is ink and the ink is applied to the article, the wells are filled with the ink while any ink on the remaining surface or land area of the article is wiped off. Since the ink is contained only in the pattern defined by the wells, it is this pattern that is transferred to another surface.
In commercial practice, a wiper or doctor blade is used to remove any excess liquid from the land area of the liquid transfer article. If the surface of the coated article is too coarse, excessive liquid, such as ink, will not be completely removed from the coarse land area of the article thereby resulting in the transfer of too much ink onto the receiving surface and/or onto the wrong place of the receiving surface. Therefore, the surface of the liquid transfer article should be smooth and the wells clearly defined so that they can accept the liquid.
Gravure-type rolls are commonly used as liquid transfer rolls. Gravure-type rolls are also referred to as applicators or pattern rolls. A gravure roll is produced by cutting or engraving various sizes of wells into portions of the roll surface. These wells are filled with liquid and then the liquid is transferred to a receiving surface. The diameter and depth of the wells may be varied to control the volume of liquid transfer. It is the location of the wells that provide a pattern of the liquid to be transferred to the receiving surface while the land area defining the wells do not contain any liquid and therefore should not transfer any liquid. The land area is at a common surface level such that when liquid is applied to the surface and the liquid fills or floods the wells, excess liquid can be removed from the land areas by wiping a doctor blade across the roll surface.
The depth and size of the wells determines the amount of liquid which is transferred to the receiving surface. By controlling the depth and size of the wells, and the location of the wells (pattern) on the surface, a precise control of the volume of liquid to be transferred and the location of the liquid to be transferred to a receiving surface can be achieved. In addition, the liquid may be transferred to a receiving surface in a predetermined pattern to a high degree of precision having different print densities by having various depth and/or sizes of wells.
Typically, gravure rolls are a metal with an outer layer of copper. Generally, the engraving techniques employed to engrave the copper are mechanical processes, e.g., using a diamond stylus to dig the depression patterns, or photochemical processes that chemically etch the depression pattern. After completion of the engraving, the copper surface is usually plated with chrome. This last step is required to improve the wear life of the engraved copper surface of the roll. Without the chrome plating, the rolls wear quickly, and are more easily corroded by the inks used in the printing industry. For this reason, without the chrome plating, the copper rolls generally have an unacceptably low life.
However, even with chrome plating, the life of the rolls is often unacceptably short. This is due to the abrasive nature of the fluids and the scrapping action caused by the doctor blade. In many applications, the rapid wear of the rolls is compensated by providing oversized rolls with wells having oversized depth. However, these rolls have the disadvantage of higher liquid transfer when the rolls are new. In addition, as the rolls wear, the volume of liquid transferred to a receiving surface rapidly decreases thereby causing quality control problems. The rapid wear of the chrome-plated copper rolls also results in considerable downtime and maintenance costs.
Ceramic coatings have been used for many years for anilox rolls to give extremely long life. Anilox rolls are liquid transfer rolls which transfer a uniform liquid volume over the entire working surface of the roll. Engraving of ceramic coated rolls cannot effectively be done with the conventional engraving methods used for engraving copper rolls. Consequently, ceramic coated rolls are generally engraved with a high energy beam, such as a laser or an electron beam. Laser engraving results in the formation of a well with a new recast surface above the original surface of the roll, such recast surface having an appearance of a miniature volcano crater. This is caused by solidification of the molten material thrown from the surface when struck by the high energy beam. Specifically, recast is coating material surrounding a laser-engraved well which was not vaporized by the energy beam and which material resolidifies.
The recast surface does not significantly effect the function of an anilox roll because the complete anilox roll is engraved and has no pattern. However, in gravure printing processes where a liquid transfer pattern is required, the recast surface causes significant problems. The major difference between a gravure roll and an anilox roll is that the entire anilox roll surface is engraved whereas with a gravure roll only portions of the roll are engraved to form a predetermined pattern. In order for the gravure roll to transfer liquid in a controlled manner determined by the pattern, fluid has to be completely wiped from the unengraved land areas by a doctor blade. Any fluid remaining on the land areas after being wiped with a doctor blade will be deposited on the receiving surface where it is not desired. With a laser engraved ceramic roll, the doctor blade cannot completely remove liquid from the land area due to the recast surfaces or porosity of the land areas which retain some of the liquid. Although the recast surfaces should be removed for most printing applications, the porosity of the land area is still a major problem since liquid can be trapped on the land area and transferred to a receiving surface. This problem is particularly severe at transition zones between adjacent wells and patterns where the liquid tends to smear onto the land areas where it should not be.
It is an object of the present invention to provide a low porosity, high density ceramic or metallic carbide coated laser-engraved liquid transfer article, such as an impression roll, which has land areas which can easily and efficiently be wiped clean of a liquid and a plurality of wells for retaining a metered amount of liquid that can be transferred to a suitable receiving surface.
Another object of the present invention is to provide a process for producing a low porosity, high density ceramic or metallic carbide coated laser-engraved liquid transfer article.